High intensity discharge lamp electrode

ABSTRACT

In horizontally operated high pressure sodium vapor lamps using alumina ceramic envelopes, spotting and blackening of the envelope has been traced to loose emission material released from the electrodes. Loss of emission material from a double layer wound tungsten electrode is prevented by wrapping a foil of refractory metal such as tungsten or molybdenum around the inner coil after the emission material has been applied, and then winding or screwing the outer coil over the foil. The outer coil holds the foil in place and the foil in turn holds the emission material in place.

United States Patent 1191 Collins et al.

May 7, 1974 HIGII INTENSITY DISCHARGE LAMP ELECTRODE Inventors: Byron R. Collins, Euclid; Charles l.

McVey, Shaker Heights, both of Ohio General Electric Company, Schenectady, NY.

Filed: Apr. 19, 1973 Appl, No.2 352,607

Assignee:

us. (:1 313/217, 313/346, 313/352 1111. c1. 11013 61/06 Field 61 Search 313/217, 346, 352

References Cited UNITED STATES PATENTS White 313/346 Primary Examinerl-lerman Karl Saalbach Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or FirmErnest W. Legree; Lawrence R. Kempton; Frank L. Neuhauser [57] ABSTRACT In {horizontally operated high pressure sodium vapor lamps using alumina ceramic envelopes, spotting and. blackening of the envelope has been traced to loose emission material released from the electrodes. Loss of emission material from a double layer wound tungsten electrode is prevented by wrapping a foil of refractory metal such as tungsten 0r molybdenum around the inner coil after the emission material has been applied, and then winding or screwing the outer coil over the foil. The outer coil holds the foil in place and the foil in turn holds the emission material in place.

9 Claims, 3 Drawing Figures BACKGROUND OF THEINVENTION The invention relates to high intensity metal vapor discharge lamps utilizing thermionic electrodes which .comprise a base or support structure of refractory metal, commonly tungsten, having open spaces or cavities therein containing electron-emissive material such as alkaline earth metal oxides or compounds, and usually provided with an overlying protective grid or winding of refractory metal.

One well-known electrode structure utilizes tungsten wire wound in two layers on a tungsten shank. An inner coil has central turns which are spread apart to leave space or cavities between turns, and an outer coil whose turns are close wound. Electron-emissive material is retained primarily in the interstices between the turns of the inner coil and the outer coil wound thereover. These electrodes are usually of the self-heating kind wherein the discharge current is the source of heat, but they may also be heated by current from an external source, particularly at starting. 3

Such electrodes are used in high intensity sodium vapor lamps of the kind described in U. S. Pat. No.

3,248,590 Schmidt, entitled High Pressure Sodium Vapor Lamp. These lamps utilize a slender, tubular envelope of light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina. The filling comprises sodium along with a rare gas such as Xenon to facilitate starting and mercury for improved efficiency. The ends of the alumina tube are sealed by suitable closure members each supporting one of the foregoing electrodes. The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with the usual screw base. The electrodes of the arc tube are connected to the terminals of the screw base, that is to shell and center contact. The interenvelope space is usually evacuated in order to conserve heat.

SUMMARY OF THE INVENTION A problem frequently encountered with lamps of the foregoing kind when they are operated horizontally is spotting and blackening of the ceramic arc tube along its underside. This is also observed-in inclined lamps, for instance lamps operated in tilted luminaires, as commonly done for street lighting. The blackening is not uniform and includes spots or concentrated areas tally or tilted only slightly. We have confirmed experimentally that the spotting and envelope darkening are due to such loose emission material which is released from the electrodes. The problem is aggravated by the use of dibarium calcium tungstate as the emission material because it is more powdery and sinters less than other materials. This makes releaseby the electrodes more likely.

In accordance with our invention, we substantially eliminate undesirable loss of emission material from electrodes comprising a refractory metal support structure having open spaces by wrapping refractory metal foil around the structure to retain the emissive material in place, and overlying the wrapping with a retaining grid or coil of refractory metal.

In a preferred embodiment, a double layer wound tungsten electrode structure is utilized wherein the inner coil has spaced apart turns providing cavities for the accommodation of emission material. A thin sheet of molybdenum or tungsten is wrapped around the inner coil in the form of a cylinder afterthe emission material has been applied. The outer coil is then screwed over the molybdenum sheet and the inner coil. The outer coil holds the thin sheet or foil in place and the foil in turn holds the emission material in place.

DESCRIPTION OF DRAWING DESCRIPTION OF PREFERRED EMBODIMENT A high intensity sodium vapor discharge lamp in which the invention may be embodied is illustrated at l in FIG. 1 and comprises an outer vitreous envelope orjacket 2 of elongated ovoid shape. The neck 3 of the jacket is closed by a remnant stem 4 having a press 5 through which extend stiff inlead wires 6, 7 connected By spectrographic analysis we have determined that the dark spots contain major concentrations of tungsten and minor concentrations of barium and calcium. These are the elements present in dibarium calcium tungstate emission material. The are tubes are generally translucent rather than transparent or clear, and casual inspection might not reveal anything. However close inspection has shown that small quantities of loose emission material frequently lie along the bottom side of the tube in lamps which are mounted horizonat their outer ends to the threaded. shell 8 and center contact (not shown) of a conventional screw base 9.

The inner'envelope or are tube 11 is made of sintered high density polycrystalline alumina ceramic per US. Pat. No. 3,026,201 Coble, Transparent Alumina and Method of Preparation, or of other lighttransmitting ceramic capable of withstanding the attack of sodium vapor at high temperatures. The ends of the tube have end closures, suitably thimble-like niobium metal end caps 12, 13 as illustrated which are hermetically sealed to the alumina by means of a glassy sealing composition.

Thermionic electrodes as shown at 1 4 in FIGS. 2 and 3 are mounted in the ends of the arc tube. The electrode comprises an inner tungsten wire coil 15 which is wound over a tungsten shank 16 crimped or welded in the end 17 of a niobium tube 18 which extends through the end cap and is welded thereto at 19. The central turns in the inner coil 15 are spread apart to such extent that there is space between turns approximately equal in widthto the wire. The electron emissive mix containing low work function metal compounds may be applied to the inner coil by painting, or alternatively by dipping the electrode in a suspension of the material. In the case of Ba CaWO it is conveniently applied as a suspension in ethanol. The material is retained primarily in the spaces between turns of the inner coil, as shown at 20.

After the suspension of electron-emissive mix has dried, a thin sheet 21 of refractory metal is wrapped around the inner coil in the form of a cylinder. The most suitable materials for sheet 21 are molybdenum and tungsten, and the former is preferred because it is lower in cost. An outer coil 22, shown as having closewound turns throughout, is then screwed over the inner coil and its molybdenum foil wrapping. In FIG. 3 the foil is shown in place wrapped around the inner coil and its filling of emissive material, and a section has been cut out of outer'coil 22 to expose the foil. The outer coil holds the molybdenum foil in place and the foil retains the emission material in place.

One electrode end, cap assembly is provided with a longer niobium tube 18 which has an opening into the interior of the lamp. It is used as an exhaust tube to introduce the gas filling and sodium mercury amalgam into the arc tube. It is then hermetically pinched off at 23 and serves thereafter as a reservoir for excess condensed sodium mercury amalgam. The corresponding niobium tube 18' at the other end of the arc tube may contain a small quantity of yttrium metal serving as a getter but it has no opening into the interior of the arc tube. In a lamp intended for vertical operation, the end cap having the long exhaust tube is always located lowermost.

The are tube is supported within the outer envelope by means of a mount comprising a side rod 25 which extends the length of the envelope from inlead 7 at the stem end to a dimple 26 at the dome end to which it is anchored by a resilient clamp 27. End cap 12 of the arc tube is connected to inlead 6 by welded band 29 and rod 30 while end cap 13 is connected to inlead 7 by welded band 31, flexible strap 32 and side rod 25. The

interenvelope space is desirably evacuated in order to conserve heat; this is done prior to sealing off the outer jacket. A getter, suitably barium aluminum alloy powder pressed into channel rings 33, is flashed after sealing in order to assure a high vacuum.

In the lamp illustrated in FIG. 1, the upper portion of the arc tube above the break is representative ofa prior art lamp and has dark spots and blackening 34 extending along one side, plus darkening at the end around the electrode. That side would have been located lowermost in horizontal operation of the lamp. The spotting and darkening are representative of that occurring using Ba CaWO emission material and electrodes not having the refractory metal foil of the present invention. ln some extreme cases, the reaction spots absorbed up to 50 percent of the transmitted light.

The lower portion of the ceramic arc tube is illustrative of the substantially complete elimination of spotting and reduction in end-darkening through the use of the electrode of the present invention.

When using an internal foil wrapping according to the invention the outer coil 22 serves as a grid holding foil 21 in place and does not itself directly retain the emission material in place. Therefore it is no longer essential to have a tight, close-wound outer coil and a considerable relaxing of tolerances as to closeness of winding is permissible. This offsets to some extent the added cost of the foil.

What we claim as new and desire to secure by letters patent of the United States is:

1. An electrode for a high intensity arc discharge lamp comprising a refractory metal support structure having open spaces therein, an electron-emissive material within said spaces, a wrapping of a refractory metal foil around said structure and covering said emissive material to retain it in place, and a grid of refractory metal overlying said wrapping and retaining it in place around said refractory metal support structure.

2. An electrode as in claim 1 wherein the refractory metal of the support structure and overlying grid is tungsten and the refractory metal foil is molybdenum.

3. An electrode as in claim 1 wherein the emissive material is essentially dibarium calcium tungstate.

4. An electrode for a high intensity are discharge lamp comprising a refractory metal shank, an inner coil of refractory metal having spaced apart central turns wound around said shank, an electron-emissive mate rial filling the spaces between said turns, a refractory metal foil wrapped around said inner coil to retain said electron-emissive material in place, and an outer coil of refractory metal wire wound over said metal foil to retain it in place around said inner coil.

5. An electrode as in claim 4 wherein the shank, the inner coil, and the outer coil are of tungsten and the foil is of molybdenum.

-6. An electrode as in claim 4 wherein the electron emissive material is essentially dibarium calcium tungstate.

7. A high intensity sodium vapor discharge lamp comprising a slender tubular ceramic envelope, a pair of electrodes sealed into opposite ends, a filling of sodium, mercury, and an inert gas within said envelope, each electrode comprising a refractory metal shank, an inner coil with spaced apart central turns wound around said shank, an electron-emissive material filling the spaces between turns of said inner coil, a wrapping of refractory metal foil around said inner coil, and an outer coil wound over said wrapping to retain it in place.

8. A lamp as in claim 7 wherein the shank, the inner coil, and the outer coil are of tungsten and the metal foil is of molybdenum.

9. A lamp as in claim 7 wherein the electron-emissive material within the electrode is essentially dibarium calcium tungstate. 

2. An electrode as in claim 1 wherein the refractory metal of the support structure and overlying grid is tungsten and the refractory metal foil is molybdenum.
 3. An electrode as in claim 1 wherein the emissive material is essentially dibarium calcium tungstate.
 4. An electrode for a high intensity arc discharge lAmp comprising a refractory metal shank, an inner coil of refractory metal having spaced apart central turns wound around said shank, an electron-emissive material filling the spaces between said turns, a refractory metal foil wrapped around said inner coil to retain said electron-emissive material in place, and an outer coil of refractory metal wire wound over said metal foil to retain it in place around said inner coil.
 5. An electrode as in claim 4 wherein the shank, the inner coil, and the outer coil are of tungsten and the foil is of molybdenum.
 6. An electrode as in claim 4 wherein the electron-emissive material is essentially dibarium calcium tungstate.
 7. A high intensity sodium vapor discharge lamp comprising a slender tubular ceramic envelope, a pair of electrodes sealed into opposite ends, a filling of sodium, mercury, and an inert gas within said envelope, each electrode comprising a refractory metal shank, an inner coil with spaced apart central turns wound around said shank, an electron-emissive material filling the spaces between turns of said inner coil, a wrapping of refractory metal foil around said inner coil, and an outer coil wound over said wrapping to retain it in place.
 8. A lamp as in claim 7 wherein the shank, the inner coil, and the outer coil are of tungsten and the metal foil is of molybdenum.
 9. A lamp as in claim 7 wherein the electron-emissive material within the electrode is essentially dibarium calcium tungstate. 